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Chapter 10-2 – Seafloor Spreading Study Guide
Ms. Grady
Mapping the Oceanic Floor
 During World War I German scientists introduced the ideas of using
sound waves to detect (to find) submarines.
 In the 1940’s during World War II, scientists began to use sound waves
to map the ocean floor.
 This is sometimes called echo sounding.
 Sound waves echo off the bottom of the ocean, so the longer the sound
waves take to return to a ship the deeper the water is.
Earth’s Spreading Ocean Floor
 Scientists discovered a large system of underwater mountains that have
a deep crack called a rift valley, running through their center known as
the mid ocean ridges.
 A lot of volcanic activity happens at the mid-ocean ridges.
 For example, lave erupts (explodes or vents) from the rift valley.
The hardened lava forms new ocean floor.
 In the 1960’s, Harry Hess proposed (suggested) a process which is known
as sea-floor spreading.
 As the lava flows sideways, it carries the ocean floor away from the
ridge in both directions.
Notice in the pictures above, Magma is forced upward and flows from the
cracks as the seafloor spreads apart.
Process: The lava becomes solid as it cools and forms a NEW seafloor.
Then when new seafloor moves away from the mid-ocean ridge, it cools and
contracts and becomes denser. The denser and colder seafloor then sinks
(goes down) which helps to form the ridge.
Evidence for Spreading
Scientists gathered information in 1968 while they were on a research ship called the
Glomar Challenger. They got information about the rocks on the seafloor.
Glomar Challenger had a drilling rig that allowed scientists to drill into the seafloor to
get rock samples.
Example of a drilling rig at sea.
What did the scientists find? Well the research concluded that the age of the rocks
become much older in samples found farther from the ridges. This is evidence for
seafloor spreading.
Faults along the Mid-Ocean Ridge
 Ocean-floor spreading helps to explain how continents drift (move apart).
As a piece of the ocean floor moves, it takes its continent (if it has one)
with it.
 Individual sections of mid-ocean ridges are straight but the ridge curves.
It curves because the straight sections are offset by think cracks knows
as faults.
 The youngest rock is near the mid-ocean ridge. As the floor spreads,
older rocks move farther away from the ridge.
Magnetic Stripes and Clues
 Earth’s magnetic field has a North and South Pole.
 A permanent record of the Earth’s magnetism remains in the rocks.
 Scientists discovered that the Earth’s magnetic poles reverse themselves
from time to time.
Magnetic alignment in the rocks
reverses back and forth over time
in strips parallel to the mid-ocean
ridges as shown in the picture to
the left. This shows further
evidence of seafloor spreading.
 Studies show that during the past 3.5 million years, the magnetic poles
have reversed themselves nine times. The pattern is identical (the same)
on both sides of the mid-ocean ridge.
 A strong polarity (separation or split) will show up as a large peak in a
baseline graph.
Class notes – Ocean Floor
The ocean floor is divided into 3 big areas: margins, basin, and ridge.
The Ridge
The ridge is an
underwater mountain
chain formed by cooling
magma that comes out
the division between
As they cool, they
smoke and are called
black smokers.
Mid Ocean Ridge
 The mid ocean ridge is the longest topographic feature (plato’s,
mountains) on Earth.
 There is always a rift valley on the top of an oceanic ridge system.
The Continental Margin
The margin is made up of the shelf, the slope and the rise.
The shelf is the submerged (hidden or lowered) part of the continent.
The slope is the area that drops off into the oceanic floor.
The rise is made up of the sand that is dropped at the bottom of the slope.
 As the older ocean floor moves away from the mid-oceanic ridges, it will
eventually move down deep into the Earth along the trenches, (a long
narrow valley on the oceanic floor).
 When the rocks are pushed deep enough, they are melted by the heat
of the Earth.
 Some of it will rise up through the crust and produce volcanoes along
the trench, but most of the molten rock will become part of the mantle.